Proper centering of a semiconductor wafer is essential during wafer processing and transport in order to avoid costly errors such as non-uniform processing and/or wafer breakage. During semiconductor device processing a wafer must be accurately centered on a wafer support platform or chuck in order to ensure the wafer will receive uniform processing across its entire face (e.g., uniformly deposited layers) or to ensure that the chuck will not be damaged. Similarly, due to the rapid decrease in mechanical tolerancing associated with continually decreasing system footprint (i.e., the decrease of a fabrication system""s lateral dimensions), a wafer must be accurately centered on the transfer mechanism to avoid breakage (caused by a wafer falling or striking a chamber component) during semiconductor device transport, and to reduce the probability of the wafer sliding off or being dropped by the transfer mechanism. In addition, for processes sensitive to crystal orientation or pattern alignment, wafers will have a notch or flat to signify the substrate""s crystal orientation or pattern alignment, this notch or flat must be aligned so as to be in the appropriate location with respect to the chamber components. Accordingly numerous wafer orienting and/or centering devices (i.e., positioning devices) exist.
A conventional wafer positioning device is disclosed in Japanese patent application number 05045948 published Aug. 23, 1994 (Japanese application number 948). The device disclosed in Japanese application number 948 employs a charge coupled device (CCD) camera installed above a rotary table which detects the image of a semiconductor wafer located on the rotary table and applies image processing to detect the position of the wafer""s center of gravity. Specifically the CCD camera takes in a circumferential of the semiconductor wafer located on the rotary table. The device then applies image processing to this frame so as to determine the orientation of the wafer.
In practice, however, CCD cameras have difficulty distinguishing the wafer from other surfaces such as the bottom surface of the chamber in which positioning occurs. Accordingly an improved wafer positioning device, which more reliably detects wafer position, is needed.
The present invention provides a wafer positioning device which reliably detects wafer position by employing a backlighting source. Specifically, the invention employs a CCD camera adapted to detect a first region which is larger than a wafer to be detected by the CCD camera, and thus encompasses the edge of the wafer to be detected, a backlighting source adapted to direct light to a second region which extends outwardly from at least the edge of the wafer to be detected by the CCD camera, and a controller operatively coupled to the CCD camera for determining the wafer""s position based on the backlighting detected by the CCD camera.
Preferably both the CCD camera and the backlighting source are mounted outside a vacuum chamber, adjacent one or more quartz windows which allow the CCD camera and backlighting source to respectively detect and emit light from and into the processing chamber. Most preferably the CCD camera and the backlighting source are located on the same side of the vacuum chamber, thus allowing them to share a common quartz window, such as that conventionally located on the top of a vacuum chamber. In such an arrangement a shield is operatively coupled to the backlighting source so that light emitted from the backlighting source is blocked from reaching the top surface of the wafer. This prevents light from reflecting off the top surface of the wafer and being detected by the CCD camera, and thus ensures that all light detected by the CCD camera is xe2x80x9cbacklightingxe2x80x9d coming from behind the wafer.
As described generally above, the shield ensures that light travels past the wafer surface and strikes the bottom surface of the vacuum chamber. The bottom surface of the vacuum chamber reflects at least a portion of the light back to the CCD camera. Any portion of the light which reflects from the bottom surface of the processing chamber toward the wafer will be blocked by the bottom surface of the wafer, and thereby prevented from reaching the CCD camera. Based on the reflected light detected by the CCD camera the controller is able to determine wafer position.
In a most preferred embodiment the shield comprises a first shield positioned to block direct light emitted by the backlighting source from reaching the top surface of the wafer, and a second shield positioned to block reflected light (e.g. light emitted from the backlighting source and reflected by chamber surfaces) from reaching the top surface of the wafer. The wafer is therefor effectively backlit, making the perimeter of the wafer easily detectable.
Wafer position detection may be further improved by roughening the bottom surface of the vacuum chamber so that light reflected therefrom will scatter. Scattering the light insures that some light will impact the bottom surface of the wafer and be blocked thereby. Scattered light therefore sharpens the outline of the wafer detected by the CCD camera.
Preferably the controller calculates both centering and alignment information; a rotatable platform which supports the wafer, rotates to position the wafer notch or flat in a desired orientation; and a wafer handler which transfers the wafer adjusts its operation (e.g., during wafer pick up or drop off) such that the wafer is accurately centered when subsequently placed within a processing chamber.